The experiments proposed in this application will establish the experimental basis for the design of a human immunodeficiency virus type l (HIV) vaccine in which HIV antigens are expressed in vivo by Venezuelan equine encephalitis virus (VEE) replicons. A cocktail of VEE replicon particles (VRP) directing the expression of simian immunodeficiency virus (SIV) MA/CA and ENV have been used successfully to protect rhesus macaques against disease induced by a high dose intravenous (iv.) challenge with a virulent swarm (E660) of SIV. The VEE replicon system affords several advantages as a vaccine vector for safe and effective immunization against HIV. Among these are: I) The vectored antigen is expressed to very high level, up to 20% of the total cell protein. 2) In vivo, VRP target lyrnphoid tissues, and dendritic cells in particular, leading to induction of both humoral and cellular immunity. 3) Neither priming nor boosting is diminished by prior immunity to VEE. And 4) VRP are safe; in over 1,000 rodents and 40 macaques, no untoward effects of VRP vaccines have been observed regardless of dose or route of inoculation. Three interrelated projects will utilize VRP in rodent and macaque models to l) determine optimal design of immunogens to be included in a vaccine, 2) explore the mechanisms whereby VRP target dendritic cells in vivo as well as the consequences of dendritic cell targeting for induction of an immune response, and 3) characterize the immunological parameters which determine protection in SIV and SHIV models. The integrated results from these projects will provide fundamental information regarding immunization with lentivirus antigens and will form the basis for an intensive program to develop a VEE replicon vaccine candidate for HIV.